1. Field of the Invention
The present invention relates to an image forming method for forming an image by electrophotography, and to a copier, facsimile device, printer, or other such image forming apparatus that makes use of this method.
2. Description of the Related Art
Conventional image forming methods for forming an image by electrophotography have been disclosed, for example, in Japanese Laid-Open Patent Applications 2002-202638 and 2002-287545. With these image forming methods, first a latent image is formed by an exposure apparatus on a latent image support such as a photoreceptor, after which this latent image is developed and made visible by causing toner to adhere electrostatically thereto. Next, this developed toner image is electrostatically transferred onto transfer paper or another such recording medium, then a fixing roller or other such heating member is brought into close contact to heat this toner and fix it to the recording medium.
One advantage to an electrophotographic image forming method such as this is that an image can be easily formed on the basis of electronic image information, but a disadvantage is that image quality is inevitably inferior to that produced by offset printing. In particular, with images having density gradation, such as photographs or pictures, the roughness is much more pronounced than with offset printing, and tends to give the viewer an impression of lower quality. Consequently, an important question with electrophotography is how to minimize this appearance of lower quality.
RMS granularity, which has been standardized in ANSI PH-2.40-1985, is known as an index of the roughness of an image, and this is calculated from the following Eq. 1.RMS granularity σD=[(1/N)×Σ(Di−D)2]1/2  Eq. (1)
Here, N is the number of data, Di is the density distribution, and D is the average density (D=1/NΣDi).
Also, granularity GS defined by Dooley and Shaw of Xerox is another known index of roughness. This is the numerical value obtained by integrating the cascade values of a visual spatial-frequency characteristic (visual transfer function (VTF)) and the Wiener Spectrum (hereinafter referred to as WS(f)). WS(f) is the squared ensemble average of a Fourier spectrum obtained by the Fourier transformation of a density fluctuation from an average density obtained by scanning an image with a microdensitometer. The granularity GS is calculated from the following Eq. 2 (for details, see Dooley and Shaw: “Noise perception in Electrophotography,” J. Appl. Photogr. Eng., Vol. 5, No. 4, (1979), pp. 190-196).granularity GS=exp(−1.8D)∫(WS(f))1/2VTF(f)df  Eq. 2
Here, D is the average density, f is the spatial frequency (c/mm), and VTF(f) is the visual spatial-frequency characteristic.
However, of the images printed out by a given image forming apparatus, some have relatively good RMS granularity σD and granularity GS, while others do not. It is therefore difficult to evaluate the performance of an image forming apparatus on the basis of the RMS granularity σD and granularity GS of a printed image. Furthermore, up to now there had yet to be adequate study into what kind of images do not have a grainy look. Plus, none of the electrophotographic image forming apparatuses on the market today allow for the reliable formation of images that do not have a low-quality appearance.